Democratic moneyman Jon Cooper was ready to stay on the sidelines this year because the email scandal and ethics conflicts made him leery of Democratic presidential nominee Hillary Clinton. But along came Donald Trump, and suddenly nothing else mattered.

The Republican nominee’s rhetoric and behavior so appalled the New York businessman that he wanted to do more than raise money. Mr. Cooper became chairman of the anti-Trump political action committee Keep America Great and its offshoot, the Democratic Coalition Against Trump.

“I see it as my patriotic duty to oppose him,” said Mr. Cooper, 62, who was a top bundler for President Obama’s campaigns. “We’ve never had to deal with a presidential nominee with as racist, misogynistic and xenophobic views as Donald Trump.”

He said that he would not have felt compelled to get politically involved this year if any other Republican had won the nomination, including arch-conservative Sen. Ted Cruz of Texas, who is a bugbear for liberals such as Mr. Cooper.

“If it were Ted Cruz, I would not be chairing the Democratic Coalition Against Cruz,” he said. “I disagree with him on virtually every issue, whether it is social or economic or what have you. But I see no indication that he is a sexist and a misogynist and a racist and a xenophobe. I see no evidence he poses the existential threat to American values the way Donald Trump does.”

Mr. Cooper’s inability to sit out the election — which he had every intention of doing after his favorite candidate Vice President Joseph R. Biden declined to run — demonstrates Mr. Trump’s power to energize his opponents.

Mrs. Clinton, who is disliked by Americans nearly as much as Mr. Trump, has a similar effect. Her run has spurred the formation of groups such as the Stop Hillary PAC.

Voters have found much to dislike about both major party nominees.

Mrs. Clinton has an unfavorable rating of 53 percent, versus 41 percent who view her favorably, while Mr. Trump had an unfavorable-favorable balance of 60 percent to 34 percent in the Real Clear Politics average of recent polls.

“It is really hard election for almost all of us,” said Barnard College political science professor Michael G. Miller. “There is a lot of evidence that there is a lot of nose-holding going on among voters and donors.”

He said the super PACs provided an ideal outlet for anti-candidate sentiment, allowing people to quickly take action and inject money into a cause. Indeed, Democratic Coalition Against Trump quickly gained traction with its digital campaign, engaging more than 3.5 million views and reaching another 16.8 million people with it’s anti-Trump message in August.

Mr. Cooper said he was offended by Mr. Trump from the start, including his harsh language directed at women and insulting remarks about Mexicans. But Mr. Trump, a billionaire real estate tycoon, went too far when he accused a federal judge of being biased because of his Mexican heritage and used a symbol resembling the Star of David in an tweet attacking Mrs. Clinton.

Early in the primary race, Mr. Cooper rebuffed overtures from Clinton supporters to lend his fundraising prowess to the former secretary of state’s run.

He said he was not satisfied with Mrs. Clinton’s explanations about her secret email setup at the State Department and potential conflicts of interest with foreign entities making donations to the Clinton Foundation and paying huge fees for speeches by former President Bill Clinton.

“It’s just the drip, drip, drip that is a little concerning, and I just wish that there would have been a more forceful response from the Clinton campaign to some of this,” he said at the time.

Mr. Cooper, who is gay and married to longtime partner Robert Cooper, also was disappointed with how long it took Mrs. Clinton to support gay marriage as a constitutional right.

“Although we have two flawed candidates, we only have one candidate who is really spitting in the face of long-treasured American values. I do not say that lightly,” said Mr. Cooper. “Whether it’s Hillary Clinton, who is less than perfect, or one of the other candidates, it will be better than Donald Trump.”

He also said that, after more than a year of scrutiny, he hadn’t seen any evidence of criminal wrongdoing related to donations to the Clinton Foundation or the speaking fees.

Mr. Cooper said that he planned to vote for Mrs. Clinton because her political views closely align with his own. He insisted, however, that he wasn’t working to get her elected.

“Whether people vote for [Libertarian nominee] Gary Johnson or [Green Party nominee] Jill Stein or write in a name, our mission is to make sure voters don’t cast their ballot for Donald Trump,” said Mr. Cooper.